How to change where Steam games install on your PC

Stop Steam from filling up the wrong drive with a few simple steps.
By default, Steam will download your games to whatever is the main drive on your PC. That is, whichever one Windows is installed on. In many cases that's not going to be a problem, but it will if you have a small SSD boot drive to complement a larger mass storage HDD, for example.
In this case, you need to tell Steam that you want things somewhere else. It's not immediately obvious how to do it, but it's not complicated, either.